It’s an interest penalty at 6% annual rate, not a flat 6%, and it’s based only on the amount short below equally paid quarterly amounts of your tax due and what you actually paid, not the total amount. I think the withholdings count equally for each quarter, so say you end up withholding $10k and paying $7.5k in Q3+Q4. Then your $25k total of 4 quarterly amounts paid might be viewed for timing purposes as Q1. $2.5k from 1/4 of withholding Q2. $2.5k from 1/4 of withholding Q3. $10k, $2.5k from 1/4 of withholding and $7.5 estimated Q4. $10k, $2.5k from 1/4 of withholding and $7.5 estimated Whereas you were “supposed” to pay $6,250 for each quarter, under an equal amount approach. So ballpark Q1 is short $4k but you catch up and overpay that extra amount in Q3 (above the regular equal amount). Likewise Q2 is short $4k and gets caught up in Q4. So you’d pay 6% on $4k for 6 months, twice, which would come to about $240 plus a little more maybe in compound interest or waiting to pay the extra $240 until April or something. But this is what I mean that it’s not going to be a big number. If you really care, you can see all the details in the instructions for Form 2210. https://www.irs.gov/forms-pubs/about-form-2210
I have quite a bit in savings so I'm going to just pay it all in full and get a refund next year. But yeah next year I might move
%% MOST any good CPA, will help you answer this question over phone, free, be polite over the phone., SteVen W. Federal tax, when i paid it quarterly, you simply owed last year [2018]estimated tax, on for example on 2019 federal tax; even if you made much more this year/ no penalty. BUT check with 4 or 5 CPAs, free over the phone. SO make sure you put the right amount[ current,2019 federal tax] in a savings or checking account until its due. I'm 100% sure you should call like 5 CPAs . I get that kind of info free; first ask how much they charge per hour, +average small business tax return. I never would pay a CPA for free phone work. I'm back to the up-trending bull Market [SPY,QQQ......]-NOT a prediction